A liquid crystal display element has advantages over other display element in terms of thinness, lightness in weight, and small power consumption, and is therefore widely used for image display devices such as a television and a monitor, and image display devices provided on: OA (Office Automation) equipments such as a word processor, and a personal computer; and information terminals such as a video camera, a digital camera, and a mobile phone.
There are conventionally well-known liquid crystal display modes for the liquid crystal display element, such as the TN (twisted Nematic) mode using Nematic liquid crystal, the display mode using FLC (ferroelectric liquid crystal) or AFLC (anti-ferroelectric liquid crystal), and the polymer dispersed liquid crystal display mode.
Among the liquid crystal display modes, for example, the TN mode using Nematic liquid crystal is adopted in the conventional liquid crystal display element, and the liquid crystal display element is in the practical use. However, the liquid crystal display adopting the TN mode has drawbacks in slow response and a narrow viewing angle. Those disadvantages are large hindrances for the TN mode to take over CRT (Cathode Ray Tube).
The mode using the FLC or AFLC allows high-speed response and wide viewing angle, but has drawbacks in shock resistance and temperature characteristics. This prevents wide and practical application of the mode using the FLC or AFLC.
The polymer dispersed liquid crystal display mode using light scattering does not need a polarizing plate, and allows high-luminance display. However, in the polymer dispersed liquid crystal display mode, it is intrinsically impossible to control the viewing angle with the use of a phase plate. Further, the polymer dispersed liquid crystal display mode has a problem in its response property. Therefore, the polymer dispersed liquid crystal display mode has only a few advantages over the TN mode.
In each of those display modes, liquid crystal molecules are aligned in a certain direction, and viewing angle depends on an angle with respect to the liquid crystal molecules. That is, in the display mode, there is restriction in the viewing angle. Further, each of the display modes uses that rotation of the liquid crystal molecules which is caused by electric field application. Because the liquid crystal molecules rotate in the alignment all together, a response speed is slow. Although the mode using the FLC or AFLC has advantages in its response speed and its viewing angle, the mode has a problem of irreversible alignment breakdown due to external force.
Apart from the display elements using the molecule rotation due to the application of the electric field voltage, an electronic polarization display mode using the Kerr electro-optic effect is proposed.
The term “electro-optic effect” indicates such a phenomenon that reflective index of a substance varies according to an outer electric field, and there are two types in the electro-optic effect: (i) the Pockels effect that is proportional to the electric field, and (ii) the Kerr effect that is proportional to square of the electric field. The Kerr effect, that is the Kerr electro-optic effect, was adopted early on in high-speed optical shutters, and has been practically used in special measuring instruments.
The Kerr effect was found by J. Kerr in 1875. Well-know materials showing the Kerr effect are organic liquid materials such as nitrobenzene, carbon disulfide, and the like. These materials are used for, for example, the aforementioned optical shutter, an optical modulating device, an optical polarizing device. For example, these materials are used, e.g., for measuring the strength of high electric field for power cables and the like, and similar uses.
Later on, Research has been conducted to utilize a large Kerr constant of the liquid crystal materials for use in light modulation devices, light deflection devices, and optical integrated circuits. It has been reported that a liquid crystal compound has a Kerr constant more than 200 times higher than that of nitrobenzene.
Under these circumstances, studies for using the Kerr effect to a display device have begun. It is expected that use of the Kerr effect attains a relatively low voltage driving than the Pockels effect that is proportional to electric field, because the Kerr effect is proportional to the square of the electric field. Further, it is expected that the utilization of the Kerr effect attains a high-response display apparatus because, e.g., the Kerr effect shows a response property of several. μ seconds to several m seconds, as its basic nature.
However, one of the problems in practically employing the Kerr effect for a display element is driving voltage that is greater than that for driving a conventional liquid crystal display element. In view of this problem, Japanese Laid-Open Patent Application Tokukai 2001-249363 (published on Sep. 14, 2001) discloses a method of subjecting the substrate surface to alignment treatment in advance so as to encourage induction of the Kerr effect.
Further, Japanese Laid-Open Patent Application Tokukaihei 11-183937 (published on Jul. 9, 1999) discloses a technique of dividing a liquid crystal material into small domains by a polymer to realize a high-speed liquid crystal optical switching element with a wide viewing angle. In the invention of Tokukaihei 11-183937, the liquid crystal material is divided into small domains with a diameter=0.1 μm or less so as to suppress the temperature dependency of the Kerr constant of liquid crystal that exhibits optical isotropy when no voltage is applied.
However, the art disclosed in the foregoing publication has a problem in that the region in which the Kerr effect can be easily generated is limited to a vicinity of surfaces of the substrate. More specifically, in the foregoing technique, the molecules are aligned only in the vicinity of the substrate boundary where the alignment treatment has been performed. Therefore, the art of the foregoing technique provides only little reduction of the driving voltage.
The small reduction in driving voltage occurs because the molecular orientation caused by the electric field application, in other words, i.e., the molecular orientation caused by the Kerr effect, has a short long-range order. That is, for example, in a TN-mode liquid crystal display device or the like, the orientation direction of the liquid crystal molecules is changed in the whole substrate in the normal direction; however, in a liquid crystal display device employing the Kerr effect, there is a little difficulty in propagating the alignment order of the molecules in the vicinity of the substrate to the inner portion (bulk portion) of the cell. For this reason, the method of the foregoing publication cannot significantly reduce the driving voltage to overcome the practical problem.
Further, when employing the method of the foregoing publication for a display element that contain negative-type liquid crystal that are aligned by voltage application in the substrate normal direction, there is a problem that the directions of major axes of the molecules in the bulk region are not consistent. More specifically, in the vicinity of the substrate boundary where the alignment treatment has been performed, the liquid crystal molecules are aligned in the rubbing direction by voltage application; however, in the bulk region which is away from the substrate boundary, the major axes of the molecules could be aligned in any directions. This is because, even though the polarization of the molecule can be aligned, it only causes alignment in the minor axis direction, as the polarization mostly exists in the minor axis direction of the molecule. In other words, even though the polarization is aligned by voltage application, the bulk area is optically isotropic when viewed from the front (substrate normal direction), and therefore it does not contribute to optical response. Therefore, in employment of the technique of the foregoing publication for this display device, optical response can be obtained only in the vicinity of the substrate boundary when a practical level voltage is applied, and the optical response in the bulk region cannot be obtained unless a driving voltage a lot higher than the practical level is applied.
Further, in the display element that includes positive-type liquid crystal molecules that are aligned by application of electric field in the substrate in-plane direction, the alignment in the bulk region is substantially unified to the direction of the electric field. However, in employment of the technique of the foregoing publication for this display device using the positive-type liquid crystal, reduction of driving voltage because of the alignment treatment can be achieved only in the vicinity of the substrate boundary. For this reason, this structure cannot attain reduction of the driving voltage to a practical level.
Further, in the invention of Tokukaihei 11-183937, the ratios of the monomer and a cross-linker are high, 16 wt % to 40 wt %, thus inducing an increase of driving voltage. Further, this publication describes that the created small domains do not have to be completely separated from each other. However, in this publication, the liquid crystal material in each small domain needs to be covered substantially completely like a micro capsule so as to individually create each domain. Otherwise, the average diameter of the small domains becomes large, and the material does not exhibit optical isotropy when no voltage is applied. Therefore, the small domains need to be substantially completely covered by a polymer or the like, thus increasing the content of the monomer.